A micro-electromechanical system (MEMS) device is a micro-sized mechanical structure having electrical circuitry fabricated together with the device by using microfabrication processes mostly derived from integrated circuit fabrication processes. The developments in the field of MEMS process engineering allow for batch production of electrostatically tiltable MEMS micromirrors and micromirror arrays that can be used in such areas of photonics as visual displays, optical attenuators and switches, sensors with optical readout, and other devices. There are at least two main micromachining techniques used to manufacture MEMS micromirror devices.
One such technique is based on so called bulk micromachining, in which the whole thickness of a silicon wafer is used for building micro-mechanical structures. Silicon is machined using various etching processes. Anodic bonding of glass plates or additional silicon wafers is used for adding features in the third dimension and for hermetic encapsulation. Complex three-dimensional mechanical structures with mirror hinges disposed under the mirrors can be created using bulk micromachining. High fill factors defined as ratios of geometrical area occupied by the micromirrors to the total area of the array, can be achieved, allowing the creation of high-performance visual display pixel arrays. Detrimentally, the bulk micromachining techniques are very complex and require many process steps.
Another technique is based on so called surface micromachining, in which layers are deposited on the surface of a substrate as the structural materials to be patterned, instead of a three-dimensional processing of the substrate itself, which significantly simplifies the manufacturing processes involved. The original surface micromachining concept was based on thin polycrystalline silicon layers patterned as movable mechanical structures and released by sacrificial etching of an underlying oxide layer. This MEMS paradigm has enabled the manufacturing of low cost MEMS devices.
New etching technology of deep reactive ion etching (RIE) has made it possible to combine good performance typical to bulk micromachining with in-plane operation typical to surface micromachining. While it is common in surface micromachining to have structural layer thickness in the range of 2 μm, in high aspect ratio (HAR) micromachining the achievable thickness of MEMS devices is from 10 to 100 μm. The materials commonly used in HAR micromachining are thick polycrystalline silicon, known as epi-poly, and bonded silicon-on-insulator (SOI) wafers. This combined technology is quickly becoming the technology of choice for manufacturing MEMS micromirror devices.
A significant problem of utilizing either type of MEMS micromirror devices in visual display systems and, or in optical switching systems is related to presence of unwanted reflections from a fraction of the MEMS substrate not covered by the micromirrors. Due to technological and construction limitations, the fill factor of MEMS micromirror arrays, defined as MEMS micromirror area divided by the total MEMS substrate area, is less than unity. Inter-mirror gaps are required to prevent the micromirrors from touching or sticking to each other, and to prevent electrical cross-talk between the micromirrors. For some devices, the gaps are also required to accommodate the micromirror hinge structures. Due to presence of said gaps between MEMS micromirrors in an optical system, a fraction of light falling onto the micromirrors leaks through the gaps therebetween and reflects from the MEMS substrate, propagating back through the gaps and into the optical system. As a result, a background light is present in the system regardless of a tilt angle of a MEMS micromirror. This background light lowers achievable contrast ratio, that is, a ratio of “white” luminosity to “black” luminosity of a picture element of a picture generated by a MEMS visual display. The degradation occurs due to raising a level of “black” luminosity due to presence of the background light mentioned above. In case of an optical switch application of MEMS, the background light lowers achievable ON/OFF ratios and increases optical crosstalk, by leaking through in the “OFF” state of an optical switch. Thus, optical performance of a MEMS micromirror device is substantially degraded.
One approach aimed at improving the contrast ratio of a visual display consists in covering an area of a substrate under the gaps between the micromirrors with a light-absorbing material. This approach, dubbed in the visual display industry as a “black grid” approach, allows one to improve the contrast ratio of a picture generated by a display. For example, U.S. Pat. No. 6,844,959 in the name of Huibers et al., assigned to Reflectivity, Inc. and incorporated herein by reference, teaches such black grid structures for a MEMS spatial light modulator. One drawback of the black grid approach is related to the fact that most of black grid materials reflect some light falling thereon. Even so called “black chrome” material frequently used as a black grid material in liquid crystal displays has a residual reflectivity of about 3%, which may not be sufficient to completely remove the background light. Another drawback is that the absorbed light causes the black grid layer to heat up which creates undesired local temperature gradients, as well as raises the overall temperature of the MEMS substrate.
Another prior-art approach consists in using reflective cusps on the MEMS substrate, disposed in the gaps between the micromirrors in the micromirror array. The U.S. Pat. No. 7,167,613 in the name of Miller et al., assigned to JDS Uniphase Corporation and incorporated herein by reference, teaches using such cusps for an optical switch application. Turning to FIG. 1, a cross-section of a prior-art MEMS micromirror device 100 with inter-mirror gap reflection suppression is shown, having a substrate 102, a cusp 104, and micromirrors 106A and 106B. A beam of light 108 falls into a gap 110 between the micromirrors 106A and 106B and reflects from the cusp 104 towards the micromirror 106A as a light beam 112. The cusp 104 prevents the light beam 112 from exiting back through the gap 110, whereby the extinction ratio is improved and the optical cross-talk is reduced. One drawback of the cusps approach is that manufacturing of oblique-tilted reflective structures is not directly compatible with planar MEMS manufacturing technologies used to manufacture the rest of the MEMS micromirror device 100.
It is therefore a goal of the present invention to provide a MEMS micromirror device effectively suppressing unwanted reflections from the MEMS substrate, thereby improving the optical performance of the MEMS micromirror device. Importantly and advantageously, the backreflection suppressing features of the MEMS device of the present invention are manufactured by a process-compatible, inexpensive and a versatile method suitable for a broad variety of types of MEMS micromirror devices. Further, advantageously, the MEMS micromirror device according to the present invention can be constructed in a variety of configurations, so as to prevent the light falling into the gaps between the micromirrors from ever exiting said gaps, by making it scatter in a well controlled and predictable fashion in a desired one or more directions.